1. Field of the Invention
The present invention provides a coal liquefaction process by which to generate and employ a superior process derived solvent to liquefy coal.
2. Brief Description of the Prior Art
In conventional coal liquefaction processes, particulate coal is treated at an elevated temperature and pressure with a solvent and either with or without hydrogen gas and either with or without a catalyst to convert the coal into liquid and low melting temperature solid products. These products also can be further up-graded through catalytic hydrogenation to produce higher quality liquids and solids.
Presently, little is known of the exact mechanisms by which the coal is transformed into soluble form or of the detailed molecular structure of coal and the soluble coal products. It is known that many coals are easily solubilized and for others solublization is more difficult. It is believed that coal may be comprised of a loose structure of molecules bonded together by hydrogen bonds and numerous weak chemical bonds. If the coal is not properly treated during the early stages of the solubilization, the coal can thermally regress to a highly refractory polynuclear structure which results in production of little of the desired distillate range products or lighter liquids. Some correlations have been made between the rank of the coal and ease of the solublization and product yield. Little is known as to how to optimize these yields and improve the product quality.
The initial products produced by a coal liquefaction process such as solvent refined coal (SRC) may have utility as a substitute clean fuel or boiler fuel. However, for substitute fuels of higher quality, particularly distillate fuels, specifications on viscosity, melting point, ash, nitrogen and sulfur contents are much more stringent. Attempts to meet these specifications by operating the coal liquefaction process under more severe operating conditions have met with many difficulties. For example, the processes have experienced low liquid and high gas yields, high hydrogen consumption, difficulty of separating residue and ash and excessive retrogressive product formation which often completely plugs process transfer lines and reactors.
Alternative methods of improving product specifications through catalytic hydrogenation also are difficult. The problems which arise include: (i) a susceptibility of the liquefaction products to retrogress and deposit on the catalyst used for their conversion; (ii) diffusion limitations of the catalyst as the molecular size of the coal liquefaction products approach the pore size of conventional catalysts; and (iii) catalyst poisoning by metal contaminants and other contaminants, such as, nitrogenous compounds, contained in the coal liquefaction products.
It is known that the composition of the products produced during a coal liquefaction process depend not only upon the nominal process operating conditions such as temperature, pressure and residence time, but also upon the solvent employed. It is known that a "good" solvent reduces the production of gases and lower quality products, such as preasphaltenes, and increases the production of higher quality distillate products. It would be desirable to provide a means by which a solvent can be produced which will upon liquefaction of the coal permit production of the desired coal liquefaction products with minimum consumption of hydrogen.